In an analyzer using an ion storage device, such as, for example, a Fourier Transformation Ion Cyclotron Resonance (FTICR) apparatus or an ion trap mass spectrometer, ions are isolated (or selected) as follows. While ions are stored in an ion storage space, an appropriate electric field is applied to the ion storage space, whereby ions having certain mass to charge ratios (m/e) are selectively ejected. Such a method that enables selection of ions while they are stored in the storage space allows the use of an advanced mass analysis called tandem mass spectrometry (MS/MS).
In the MS/MS analysis, ions of various mass to charge ratios are given from an ion generator to an ion storage space. When a certain selecting electric field is applied to the ion storage space, only ions of a specific mass to charge ratio remain in the ion storage space and the other ions are ejected. Then another electric field is applied to the ion storage space to fragment the remaining ions (precursor ions), whereby fragmented ions of the precursor ions are generated in the ion storage space. When an appropriate device operating parameter (or parameters) is changed, the fragmented ions in the ion storage space are ejected toward the ion detector, so that a mass spectrum of the fragmented ions of the precursor ions is obtained.
Since the mass spectrum of the fragmented ions include the structural information of the precursor ion, the MS/MS analysis enables the determination of the structure of the precursor ion which could not be determined solely by measuring its mass to charge ratio (simple MS analysis). For ions having more complex internal structures, repeating selection and fragmentation several times (MSn analysis) is effective in revealing them.
The ion selecting electric field is normally produced by applying voltage waves of opposite polarities to the opposing electrodes defining the ion storage space without changing the ion storing condition. Especially in an ion trap mass spectrometer, voltage waves of opposite polarities are applied to the two end cap electrodes of the ion trap when ions are selected, while an RF voltage applied to the ring electrode, which is independent of the voltages applied to the two end cap electrodes, keeps storing ions in the ion trap space surrounded by the ring electrode and the two end cap electrodes. Ions stored in the ion storage space oscillate with their characteristic frequencies which correspond to their mass to charge ratios. When an appropriate ion selecting electric field is applied there, the oscillation of the ions is modulated. If the ion selecting electric field includes the component frequency near the resonance frequency of the ions stored in the ion storage space, the ions resonate with the component frequency and their oscillation amplitude becomes larger. In the meantime, such ions collide with the electrodes surrounding the ion storage space or escape from the opening (holes) of the electrodes, so that they are lost from the ion storage space. In an ion trap mass spectrometer, the characteristic frequency of an ion is different in the axial direction and in the radial direction, and, normally, the axial oscillation is used to expel ions in the axial direction.
For the ion selecting wave, a Stored Waveform Inverse Fourier Transform (SWIFT) wave or a Filtered Noise Field (FNF) wave is often used. SWIFT is described in U.S. Pat. No. 4,761,545, and FNF is described in U.S. Pat. No. 5,134,826. A SWIFT wave or a FNF wave is composed of many component sinusoidal waves of various frequencies, but lacks a component at or around a certain frequency (“notch frequency”). The intensity of the ion selecting electric field is determined so that the ions resonating with the component waves are all ejected from the ion storage space. In this case, ions having the resonance frequency corresponding to the notch frequency do not resonate and are not ejected from the ion storage space. Thus only those ions remain in the ion storage space, and selection of ions is achieved.
Actually, even if the frequency of the applied electric field is slightly different from the characteristic frequency of ions, the ions can be excited by the electric field and its amplitude of oscillation increases. Thus a notch is set to have a certain width. But ions having the characteristic frequency at either end of the notch oscillate uncontrollably, so that some of the ions are ejected and some remain in the ion storage space depending on the intensity of the electric field.
Since the characteristic frequency of an ion changes due to the space charge around the ion, it changes due to the number of ions stored in the ion storage space. Thus, when a high-resolution ion selection is aimed for by using a narrow notch width, some part of the object ions may be ejected. In this case, ion selecting waveform having a wide notch is first used to expel ions having characteristic frequencies apart from the object frequency, so that the amount of ions stored in the ion storage space is decreased. Then another ion selecting waveform having a narrow notch is used to select object ions at high resolution. Such a method is described in the U.S. Pat. No. 5,696,376. According to the method, first, low-resolution SWIFT or FNF waveforms having a wide notch is applied to preliminarily select ions. Then another ion selecting waveform having a narrower notch width for attaining a desired resolution is applied to the remaining ions. This assures stable separation efficiency irrespective of the amount of ions initially involved. But it is necessary to take enough cooling time after the preliminary selection to wait for the oscillation of ions to subside.
Conventionally, when ions are intended to be selected at a high resolution, ion selecting waveforms of different notch widths are prepared, and the amplitude of each waveform had to be appropriately set. It required a long time to calculate and generate the waveforms and to appropriately adjust and control their amplitudes. As described above, enough time was necessary for cooling the ions after a preliminary selection.